1. Technical Field
The present invention relates to a light-emitting device, a method of producing a light-emitting device, an exposure unit, and an electronic device.
2. Related Art
Recently, organic EL devices using electroluminescence have widely attracted attention because of their excellent features such as high visibility due to self-emission, a small thickness and light weight, and excellent impact resistance. In such an organic EL device, a positive electrode composed of a transparent conductive material such as indium tin oxide (ITO) or tin oxide (SnO2), a hole injection layer composed of a doped polythiophene derivative, a luminescent layer composed of a light-emitting material such as polyfluorene, and a negative electrode composed of a metal, such as Ca, or metal compound having a low work function are sequentially laminated on a transparent substrate, such as a glass substrate, having thin-film transistors, which are switching elements disposed thereon.
When a low-molecular-weight material is used as the material of these organic thin films, these thin films are generally formed by vacuum deposition. However, it is difficult to uniformly form the thin films over a wide area by vacuum deposition. Furthermore, since an expensive vacuum apparatus is required and the utilization efficiency of the materials is extremely low, it is difficult to reduce the cost. Consequently, a method in which a polymeric material is used as the material of the organic thin films and the organic thin films are formed by a liquid phase method such as spin coating, dipping, or ink-jetting has been widely employed. A description will now be made of a method of forming organic thin films (a hole injection layer and a luminescent layer) by (1) al spin-coating method and (2) an ink-jet method.
(1) Spin-Coating Method
When a hole injection layer and a luminescent layer are formed by a spin-coating method, first, a plurality of pixel electrodes are formed on a substrate, and an inorganic bank layer composed of SiO2 or the like is then formed so as to separate the pixel electrodes from each other. Subsequently, the hole injection layer is formed by spin coating on the entire surface of the pixel electrodes including the inorganic bank layer. The luminescent layer is then laminated on this hole injection layer (see JP-A-2003-45665 and JP-A-2001-18441).
(2) Ink-Jet Method
When a hole injection layer and a luminescent layer are formed by an ink-jet method, first, an inorganic bank layer composed of an inorganic material such as SiO2 or TiO2 is formed so as to separate the pixel electrodes from each other. An organic bank layer composed of an acrylic resin or a polyimide resin is then formed on the inorganic bank layer. A lyophilicity-providing treatment is performed on the side walls of the inorganic bank layer and on the electrode surfaces of the pixel electrodes, and a liquid-repellency-providing treatment is performed on the inner walls and the ton surface of the organic bank layer. The hole injection layer is formed in areas separated by the inorganic bank layer and the organic bank layer thus formed, and the luminescent layer is laminated on the hole injection layer (see JP-A-2001-18441, JP-A-2003-19826, Japanese Patent No. 3,328,297, and JP-A-2003-282244).
However, the formation of a hole injection layer and a luminescent layer by (1) the spin-coating method or (2) the ink-jet method causes the following problems.
(1) In the case of the spin-coating method, a material having a low resistance, such as a PEDOT/PSS (Baytron P, manufactured by H. C. Starck), is used as the material of the hole injection layer. As described above, since the hole injection layer is formed on the entire surface of the substrate, holes in the hole injection layer are also transferred to the hole injection layer located between pixels other than luminescent areas separated by the inorganic bank layer, thereby causing cross talk. Accordingly, the hole injection layer disposed on the inorganic bank layer between pixels may also function as an electrode. Consequently, light is emitted in areas other than the luminescent areas, resulting in a problem of a degradation of display quality, i.e., blurring of the outline of luminous shapes of objects being displayed. In addition, when the hole injection layer or the like is formed on the entire surface of the substrate by the spin-coating method, a step of removing the layer disposed on mounting portions and areas for forming a negative electrode is required.
(2) In the ink-jet method, in consideration of the accuracy of photolithography, the organic bank layer is formed inside the inorganic bank layer so that a certain space is formed between the peripheries of the inorganic bank layer and the side faces of the organic bank layer. Accordingly, the laminated portion of the inorganic bank layer and the organic bank layer forms a step. This inorganic bank layer is lyophilic. Therefore, when a hole injection layer-forming material is discharged onto areas separated by the inorganic bank layer and the organic bank layer, the hole injection layer-forming material is also disposed on the area of the inorganic bank layer forming the step. The luminescent layer is then formed on this hole injection layer. In this case, the thicknesses of the hole injection layer and the luminescent layer formed on this inorganic bank layer forming the step are different from the thicknesses of the hole injection layer and the luminescent layer formed on each pixel area. Accordingly, the emission luminance in the areas surrounded by the bank is different from that in other areas.
After the light-emitting material is discharged onto the hole injection layer, a drying process is performed to vaporize a solvent in the light-emitting material, thus forming the luminescent layer. However, points (pinning points) at which drying of the light-emitting material starts are not always the same position on the side walls of the organic bank layer. The position at which the drying is started tends to be varied. Because of these variations in the pinning points on the side walls of the organic bank layer the thickness of the luminescent layer may be different between the central portion and the peripheral portion of the luminescent layer. Therefore, a luminescent layer having a uniform thickness cannot be formed. Consequently, the emission luminance is disadvantageously different between the central portion and the peripheral portion in pixel areas, and the emission lifetime is also different.